Secondary sources of current make it possible to accumulate, store and release electric power to an external electric circuit. Among these secondary sources are conventional batteries, conventional capacitors and electrochemical capacitors (also called Supercapacitors or Ultracapacitors).
An electrochemical supercapacitor usually comprises a hermetically sealed housing filled with an electrolyte, a positive electrode (cathode) and a negative electrode (anode) placed inside the housing, a separator, such as a membrane that separates the anode space from the cathode space and special lead terminals coupling the supercapacitor to external electric circuits.
Electrochemical supercapacitors are based on the capacitive (not battery type) or Faradic (battery type) method for storing electrical power. In the first case the capacity of the double electrical layer formed at the electrolyte/electrode boundary is used for energy storage. Typically carbon materials having a large specific surface are used in capacitive devices as electrodes. During the charge/discharge process in a capacitive device no chemical or phase changes take place on the electrode surface or inside the electrode.
In the devices of the second type the charge/discharge process is accompanied by redox reactions on the surfaces of the electrodes. In contrast to batteries, these processes take place inside a thin layer of an electrochemically active compound on the surface of the electrodes. In the most well known supercapacitors of this type the surface of the electrodes is covered with metal oxides
Typically both methods of energy accumulation are implemented in real devices, however, it is always possible to tell which method makes the main contribution to the energy accumulation process and to classify the capacitor by the energy storing method.
Electrochemical supercapacitors exhibit a very high specific power up to as high as 10 kW/kg and a long service life of up to 1 million charge/discharge cycles. These characteristics provide for a wide range of potential applications for such electrochemical supercapacitors.
Nevertheless, known electrochemical supercapacitors have disadvantages, of which the most important is their low specific energy capacity. The value of the specific energy capacity of commercially available products is within the range of 1-10 W·h/kg.
The maximal values of the specific energy capacity of about 30 W·h/kg were claimed for the electrochemical supercapacitors of the Faradic type comprising carbon electrodes with ruthenium oxide on their surface. However, the high cost of ruthenium hinders the wide application of such devices.
The above-described disadvantages are based on objective factors. The maximal values of the specific energy capacity of the known supercapacitors are limited primarily by the nature of the metal oxides—the materials used in electrode manufacture, which also contributes to the high cost of these devices.
In an international application “Polymer-modified electrode for energy storage devices and electrochemical supercapacitor based on said polymer-modified electrode”, WO03065536A2, a chemically modified electrode comprises a conducting substrate and a layer of an energy-accumulating redox polymer deposited onto the layer. The redox polymer is a stack-type polymer complex compound of a transition metal having at least two different degrees of oxidation. The redox polymer also comprises monomer fragments of a planar structure with a branched system of conjugated π-bonds and the deviation from the plane of no greater than 0.1 nm.
A polymer metal complex with a substituted tetra-dentate Schiff's base may be used as the polymer complex compound of a transition metal—for example, the compound from the group: poly-[Me(R, R′-Salen)],
where: Me—transition metal;
                Salen—residue of bis-(salicylaldehyde)-ethylenediamine in Schiff's base;        R—H or electron-donating substituent, for example, CH3O—, C2H5O—, HO— or —CH3;        R′—H or Hlg,        
may be used as said polymer metal complex.
Energy-accumulating devices with electrodes comprising redox polymers can possess the properties of different types of supercapacitors and of electrochemical batteries, i.e. a high specific power and high specific energy capacity, respectively. The main distinguishing feature of the known electrochemical capacitor is the design of its electrodes—at least one of them is made as a chemically modified electrode.
It should be noted that the examples given in the international application WO03065536A2 don't exhaust the possible ways of using the polymer metal complexes with substituted tetra-dentate Schiff's base.